The present invention relates to a system for imaging defects in a material of a plate- and layer-shaped structure by area-scanning an ultrasonic beam through a liquid medium and, more particularly, to an ultrasonic imaging system having an arithmetic operation control unit suitable for the high resolution imaging of a material of a distorted surface.
A conventional ultrasonic flaw detector system to three-dimensionally scan a test sample having a curved shape has been disclosed in JP-A-46-37318. This system discloses a basic method in which phases of transmission signals of a plurality of adjacent transmitting elements are so controlled that a focal point is formed in the sample and the image is displayed by the echoes from the focal point zone. However, no consideration is give to a method of controlling the focal point to an arbitrary depth under the surface of the sample. On the other hand, the principle that the focal point can be moved has also been known in JP-A-57-141549. However, this method has a drawback such that when the distance to the surface (incident point to a sample) changes, the focal point moves and this movement is not linear due to the difference of the sound velocities. Thus, this movement cannot be corrected by known means.
When a bonding zone of a layered structure of a test sample is extended along the imaging surface of the sample, in general, the distance from the imaging surface to the bonding zone to be imaged (hereinafter, referred to as an imaging object zone) is approximately known from a design value or the like. However, when the imaging surface is distorted or is a curved surface or when the imaging surface is inclined although it is flat surface, the distance in the liquid medium from the ultrasonic irradiating surface of the ultrasonic converter to the imaging surface (hereinafter, this distance is referred to as a medium distance) is not constant. The sound velocities in the liquid medium and in the sample differ. Therefore, the movement of the focal point is not linear. It is difficult to control the movement of the focal point so as to focus onto the imaging object zone. For instance, when the liquid medium is water and the sample is a silicon crystal plate, the underwater sound velocity v.sub.w =1.5 mm/.mu.sec and the sound velocity v.sub.sl of the longitudinal wave in silicon is v.sub.sl =8.4 mm/.mu.sec and v.sub.sl is 5.6 times as large as v.sub.w. Therefore, even if the distortion of the imaging surface is about 20 .mu.m, an error of about 110 .mu.m occurs in silicon. Thus, the error is magnified.